Selected heat treatment and hardening method for nails

ABSTRACT

A selectively hardened nail includes a nail head, an elongated shank, and a tip. A portion of the nail shank is made harder than the remainder by selectively heat treating that portion, and cooling it. The nail has controlled bending properties. The selectively hardened portion does not significantly bend when the nail is fired at a surface using a power driving tool, or during use of the nailed object.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of patent application Ser. No.09/229,527, filed on Jan. 13, 1999, now U.S. Pat. No. 6,086,305.

FIELD OF THE INVENTION

This invention relates to nails and similar elongated fasteners havingportions selectively hardened to create desired properties and behavior,and a method of making the selectively hardened nails.

BACKGROUND OF THE INVENTION

Various techniques are known for treating the surfaces of nails andsimilar elongated fasteners. U.S. Pat. Nos. 5,655,969 and 5,564,876,both issued to Lat, disclose nails manufactured from carbon steel wirepre-coated with a protective metallic layer having corrosion resistantproperties. The nails also have a protective corrosion-resistant cap.U.S. Pat. No. 5,178,903, issued to Lat et al., discloses coating a nailwith zinc, followed by a polyurethane resin.

U.S. Pat. No. 4,486,248, issued to Ackert et al., discloseswear-resistant railroad rails produced by controlled forced cooling fromabove the austenite transition temperature, to yield a fine perlitemetallurgical structure in the head portions of the rails.

One type of nail used in the construction industry is a positiveplacement nail. Positive placement nails are typically driven with ahigh powered driving tool to hold a 12-gauge or other steel plate towood. The nails are driven through holes present in the steel plate, andinto the wood. This construction method is used for buildingfoundations, floor joists, and roofing. Some positive placement nailsare hardened in a batch process to improve their strength. This process,which involves heat treating and hardening the entire length of thenail, is expensive and time consuming, and difficult to control.

The holes in the steel plate may only be slightly larger than the shankdiameter of the nail, and are smaller than the head diameter of thenail. When the nails miss the holes and hit the steel plate at highspeed, they become bent or distorted at the contact end and ricochet.Nails which ricochet create obvious hazards for the tool operator andothers in the area.

Pallet nails used in the crating and construction industries holdpallets together. Pallet nails are generally long and thin so as not tosplit the hardwood boards of the pallet. A pallet nail with a largershank diameter has a greater tendency to split the pallet boards duringdriving than a nail with a smaller shank diameter. If a pallet nail istoo thin, it bends easily, especially at the junction between the toppallet board and the skid, which is typically about 0.5 inch below thenail head. This bending often occurs during severe service of a pallet,when the fork lift bumps into the side of the pallet creating stressbetween a pallet board and skid. It has been difficult to develop apallet nail which minimizes board splitting and resists bending.

SUMMARY OF THE INVENTION

The present invention provides nails which are hardened only at selectedlocations for the purpose of improving performance in their specializeduse. In one embodiment, only the pointed end and lower shank portion ofthe nail is made harder than the upper portion of the shank. In anotherembodiment, a central region of an elongated nail shank is made harderthan portions of the shank above and below the region. Other embodimentsare also contemplated.

The selective hardening is accomplished by heating a portion of the nailto a temperature higher than about 1400° F., and then quenching (i.e.rapidly cooling) it. The selective heating causes that portion of thenail to transform from a ferritic pearlitic structure to an austeniticstructure. The quenching transforms the austenitic structure to amartensitic structure, which is harder and less ductile than theuntreated structure. The untreated portion of the nail has a ferriticpearlitic structure, which is softer and more ductile. The treatedportion of the nail is more resistant to bending.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a positive placement nail of the prior art, bentbecause of firing into a portion of a steel plate without a hole.

FIG. 2 illustrates a positive placement nail of the invention, whoselower end is selectively hardened.

FIG. 3 illustrates how the positive placement nail of FIG. 2 may bendwhen fired into a portion of a steel plate without a hole.

FIG. 4 illustrates a pallet nail of the prior art, bent at its upper endbetween the top pallet board and the skid, because of severe service ofthe pallet.

FIG. 5(a) illustrates a pallet nail of the invention, having an upperportion selectively hardened.

FIG. 5(b) illustrates another embodiment of a pallet nail of theinvention, having a selectively hardened portion below its upper end, toprevent bending when stress is applied between a pallet board andadjacent skid.

FIG. 6 is a top view of a heating apparatus for making selectivelyhardened nails.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

FIG. 1 illustrates a positive placement nail 10 of the prior art, havinga head portion 12, an elongated shank 14, and a tip 16. These nails aretypically driven into wood and other foundation materials through narrowpre-formed holes in a support plate made of relatively heavy (e.g.12-gauge) steel, or a similar heavy metal material. The holes in thesupport plate are slightly larger than the diameter of nail shank 14,and smaller than the diameter of head 12. The nails 10, which are firedusing a power driving tool, may miss the openings and strike the steelplate. When this happens using a typical, non-hardened nail, the tip ofthe nail is not hard enough to penetrate the steel. The shank 14 maybend in a semi-circular fashion as shown in FIG. 1, and the nail 10 mayricochet or skip at high speed, creating a hazardous workingenvironment. In some prior art nails, to make the tips sufficiently hardto penetrate the steel, the entire nails were hardened in an expensive,time-consuming off-line batch process.

FIG. 2 illustrates a positive placement nail 10 of the invention inwhich the lower portion 15 (including part of shank 14 and the tip 16)have been selectively hardened. The selective hardening is accomplishedby selectively heating the lower portion 15 to a temperature of at leastabout 1400° F., preferably about 1500-2000° F., most preferably about1600-1800° F., and then quenching (rapidly cooling) the nail. Theselective heating causes the heated portion of the nail, which istypically made of carbon steel, to transform from a ferritic pearliticmetallurgical structure to an austenitic metallurgical structure.

The heated nail is quenched using an aqueous bath or spray, anair-assisted spray or another suitable quenching medium. The quenchingcauses the previously heated portion 15 to transform from an austeniticmetallurgical structure to a martensite structure, which is harder andless ductile than the original structure. The nail may be quenched bydipping or dropping the entire nail into the quenching fluid, or byaiming the quenching fluid at the nail or at only the heated portion ofthe nail. The temperature of the quenching fluid is desirably about40-200° F., preferably about 50-150° F., more preferably about 60-100°F. Tap water or other process water is a suitable quenching medium.Other quenching media can also be employed, including without limitationprocess oil. The quenching time need not be more than about 30 seconds,and is preferably about 3-10 seconds. The nails may still be warm (up to800° F.) after removal from the quenching medium, in order to facilitatedrying.

The resulting selectively treated nail has different hardnesses atdifferent axial positions along the nail. The nail has a lower shankportion 15 (including tip 16) which is harder at that axial positionthan the remainder of the shank 14 at other axial positions. Theselectively hardened axial portion may have a Rockwell “C” (“R_(C)”)hardness value of at least about 45, preferably at least about 50, morepreferably at least about 55. The remaining axial portion of shank 14may have an R_(C) value at least about 10 units lower than the R_(C)value of the hardened portion 15. Desirably, the untreated axial portionof shank 14 may have an R_(C) value of about 35 or less.

The axial portion 15 of the nail which is selectively hardened(including the tip 16) may have a length of about 0.10-1.0 inch,preferably about 0.25-0.80 inch, more preferably about 0.50-0.70 inch.The nail 10 may have an overall shank length (including tip) of at leastabout 1.25 inch, preferably at least about 1.5 inch, more preferablyabout 1.75-3.50 inch. Preferably, the length of the selectively hardenedaxial portion 15 (including tip 16) will not exceed 50% of the totallength of shank 14 (including tip 15), in order to reduce the energycost compared to conventional methods of heat treating an entire nail.

The nail 10 may be constructed of carbon steel or another hardenablematerial. The carbon steel may have a carbon content of about 0.08-0.50%by weight. Carbon steel having a carbon content of about 0.15-0.45% byweight is desirable, with a range of about 0.20-0.40% being preferredfor the positive placement construction nails.

The selectively hardened positive placement nail 10 of the inventionexhibits a controlled bending when misfired into a solid portion of asteel support plate. This controlled bending is illustrated in FIGS.3(a) and 3(b). The selectively hardened nail 10 does not assume thesemi-circular configuration associated with prior art non-hardened nailsthat ricochet when misfired. Instead, when the selectively hardened nail10 strikes a solid steel plate at high speed, the lower hardened portion15 is sufficiently hard to penetrate the steel, and remainssubstantially straight and free of bending. The nail shank 14 bendssharply in at least one location, and possibly two locations, in thesofter portion above the hardened portion 15. The energy and momentum ofthe misfired nail is absorbed into the sharp bending of the softerportion of the nail shank.

In contrast, the shank of a fully hardened prior art nail will not bend.If a fully hardened nail is misfired into the steel plate, the nail mayonly partially penetrate, causing the tool to bounce back or recoil dueto additional stress created. The partially hardened nail of theinvention also reduces or eliminates the ricocheting of misfirednon-hardened nails, resulting in a safer working environment.

The selectively hardened nail also exhibits better holding force duringshear than a nail which is completely heat treated and hardened. When ashear force is applied to move the above-mentioned steel cover platelaterally along the surface of the wooden substrate, or vice versa, theselectively heat treated nail may bend in the non-hardened regions. Theholding power of the nail is maintained because the top portion of thenail is not disturbed, and stays in full contact with wood to aid inholding against the shear force. A fully hardened nail, by comparison,cannot readily bend. Accordingly, the entire length of the nail isreleased from the wood when the shear force causes enlargement of thenail hole.

FIG. 4 illustrates a pallet nail 20 of the prior art, having a headportion 22, an elongated shank 24, and a tip 26. These nails aretypically driven into hardwood boards used to make pallets and crating.These types of wood boards have a propensity for splitting. Pallet nailsare typically long and narrow, and typically have one or more threadedregions 28. The nails must have enough length and column strength toadequately penetrate the wood, yet be small enough in diameter so as notto split the pallet boards during driving. A common problem with palletnails is that they sometimes bend about 0.5 inch below the head portionduring abuse of the pallet, as shown in FIG. 4. This bending resultsfrom stress at the junction between a pallet board and adjacent skid,and renders the nails less effective for maintaining the integrity ofthe pallet.

In order to strengthen the pallet nails in the regions which typicallybend, the selective hardening technique described above may be appliedto the upper axial portions of the pallet nails. FIG. 5(a) illustratesone embodiment of a pallet nail in which an upper region 25 of the shank24, extending below the head 22, has been selectively heat treated andhardened. FIG. 5(b) illustrates another embodiment in which theselectively heat treated and hardened region 25 is located at a distancecentered about 0.5 inch below the head 22, but still in the upperportion of shank 24. In FIG. 5(b), the intermediate hardened region 25has untreated softer regions above and below on the shank 24. As shownin FIG. 5(b), the hardened region 25 corresponds to the junction betweena typical pallet board 27 and adjacent stringer board 29.

The process for making the selectively heat treated pallet nails isessentially the same as for the selectively heat treated positiveplacement nails, described above, with the main difference being thatthe heat treatment is applied to different regions on the nails. Theheating temperatures and quenching techniques are substantially thesame, but may vary depending on various parameters such as the desiredhardness of the resulting nail part or the carbon content of the steel.The pallet nails 20 may also be constructed from carbon steel or anothersuitable material. Carbon steel having a carbon content of about0.08-0.50% by weight is useful, while a range of about 0.15-0.45% isdesirable, and a range of about 0.18-0.35% is preferred for the palletnails.

Again, the treated axial region 25 should have a Rockwell “C” (“R_(C)”)hardness at least about 10 units higher than the R_(C) values of theuntreated axial region or regions on the shank 24. For instance, thetreated region 25 may have an R_(C) value of at least about 40,preferably at least about 45, more preferably at least about 50. Theuntreated region or regions may have an R_(C) value of about 35 or less.

Pallet nails typically range from about 1.25-3.50 inches in length. Thediameters are typically narrow, and range between about 0.075-0.150inches. By providing selectively hardened pallet nails that will notbend near the top during pallet use, the pallet nails can be providedwith smaller shank diameters without sacrificing other desirableproperties, such as bending resistance. This not only permits costsavings, but also improves the nail performance because smaller diameterpallet nails are less likely to split the wooden substrate, and areeasier to fully drive into the substrate.

Referring to FIGS. 5(a) and 5(b), the selectively hardened region 25 mayhave a length of about 0.20-1.0 inch, preferably about 0.30-0.75 inch,more preferably about 0.40-0.60 inch. The selectively hardened portion25 may commence just below the head 22 (FIG. 5(a)), or within about 0.40inch down from the head 22 (FIG. 5(b)). Preferably, the selectivelyhardened portion 25 commences about 0.20-0.30 inch below the head 22.

FIG. 6 schematically illustrates an apparatus 100 useful for heattreating selected axial portions of a large number of nails on acontinuous basis. Flammable gas from a source (not shown) enters theapparatus and is injected into a semi-circular manifold 108 located on abase 102. Firing burners 110 receive flammable gas from the manifold108. Firing burners 110 each include a nozzle which fires burning gasoutward toward a semi-circular exhaust chamber 116.

A central disk 118 having a toothed outer periphery is rotatedimmediately above the firing burners 110. A chain 124 is locatedradially outward of the disk during rotation of the disk forapproximately 270°. The chain 124 and outer periphery of disk 118 travelat the same speed and hold nails 30 in a substantially verticalorientation in front of burners 110. When not in contact with disk 118,the chain 124 passes around a series of sprockets remote from the disk.

The nails 30 enter the furnace via inlet conveyor 122, whereupon theyare inserted into openings between disk 118 and chain 124, and arecaptured between the disk and chain. As the disk 118 travels in thecircular path, a selected portion of each nail 30 is exposed to, andheated by the burners 110. After passing the last burner 110, the nails30 pass to the exit chamber 126, whereupon they are ejected andquenched. In a preferred embodiment, nails 30 are dropped into a bath ofturbulent water where they are quenched for a short time, and thenremoved for further processing.

The carrier disk 118 and chain 124 may define over 100, and possiblyseveral hundred linkage openings. Thus, the apparatus 100 mayselectively heat a large number of nails on a continuous basis, to veryhigh temperatures. By varying the size and positions of the burners 110,the apparatus 100 can be used to heat select narrow portions of thenails, or wider portions, and may be used to heat upper, lower orintermediate portions.

While the embodiments of the invention disclosed herein are presentlyconsidered preferred, various modifications and improvements can be madewithout departing from the spirit and scope of the invention. Forinstance, the selective hardening may be advantageously employed inother fastener types, including without limitation corrugated fasteners,framing nails, steel stud nails, and staples. The scope of the inventionis indicated by the appended claims, and all changes that fall withinthe meaning and range of equivalents are intended to be embracedtherein.

We claim:
 1. A method for selectively hardening a nail, comprising thesteps of: providing a nail including a nail head, an elongated shank anda tip, the elongated shank having a Rockwell “C” hardness of not morethan about 35; transporting the nail to a heating assembly; selectivelyheating an upper part of the nail shank below the nail head to atemperature of at least about 1400° F.; and applying a quenching fluidto provide a selectively hardened nail.
 2. The method of claim 1,wherein the upper part of the nail shank below the head is selectivelyheated to about 1500-2000° F.
 3. The method of claim 1, wherein theupper part of the nail shank below the head is heated to about1600-1800° F.
 4. The method of claim 1, wherein the quenching fluidcomprises water.
 5. The method of claim 1, wherein the selective heatingis accomplished by selectively applying aflame to the upper part of thenail shank below the nail head.
 6. The method of claim 1, wherein theselectively hardened nail has a Rockwell “C” hardness of at least about45 in the upper part of the shank and not more than about 35 in a lowerpart of the shank.
 7. A method for selectively hardening a nail,comprising the steps of: providing a nail including a nail head, anelongated shank and a tip, the elongated shank having a Rockwell “C”hardness of not more than about 35; transporting the nail to a heatingassembly; selectively hardening an intermediate part of the nail shankto a temperature of at least about 1400° F.; and applying a quenchingfluid to provide a selectively hardened nail.
 8. The method of claim 7,wherein the intermediate part of the nail shank is selectively heated toabout 1500-2000° F.
 9. The method of claim 7, wherein the intermediatepart of the nail shank is selectively heated to about 1600-1800° F. 10.The method of claim 7, wherein the quenching fluid comprises water. 11.The method of claim 7, wherein the selective heating is accomplished byselectively applying a flame to the intermediate part of the nail shank.12. The method of claim 7, wherein the selectively hardened nail has aRockwell “C” hardness of at least about 45 in the intermediate part ofthe nail shank and not more than about 35 in other parts of the nailshank.
 13. A method of selectively hardening a nail including a nailhead, an elongated shank and a tip, comprising the steps of: heating anaxial part of the nail shank to at least about 1600° F. without soheating other axial parts of the nail shank; and quenching the axialpart of the nail shank that was heated.
 14. The method of claim 13,wherein a lower axial part of the nail shank and the tip are selectivelyheated.
 15. The method of claim 13, wherein an upper axial part of thenail shank is selectively heated.
 16. The method of claim 13, wherein anintermediate axial part of the nail shank is selectively heated.
 17. Amethod of selectively hardening a nail including a nail head, anelongated shank and a tip having a ferritic pearlitic metallurgicalstructure, comprising the steps of: selectively heating part of the nailto transform it to an austenitic metallurgical structure; and quenchingthe part that was selectively heated to transform it to a martensitemetallurgical structure.
 18. The method of claim 17, wherein an upperpart of the nail shank is selectively heated.
 19. The method of claim17, wherein an intermediate part of the nail shank is selectivelyheated.
 20. The method of claim 17, wherein a lower part of the nailshank is selectively heated.